A decent roundup of ways that you can fine-tune your word/character-level typography on the web — I don’t 100% agree with all of his suggested specific fixes, but knowing how one can make these adjustments in the first place is the important part. I might also add mention of the CSS overflow-wrap: break-word rule which has come up twice as something I needed in the last couple of projects I worked on (okay, that’s not really a spacing technique so much as a fix for overly-long words, but thought it worth mentioning alongside font tweaks)
Notes about whitespace
Not only is every letter an object, but every space between every letter is also an object. Every space between words, every space between lines—every bit of white space is an object. When typesetting, a printer has to think about negative space as something tangible.
A nice essay from The Atlantic on the development of the humble metal (or wood) block known as the en, the invisible assistant of legibility in classic printed text.